1. Field of the Invention
This Invention relates to a method for soil remediation, and in particular to a process for removing hydrocarbon contaminants from soil, and providing a matrix suited for brine and heavy metal leaching.
Most countries have oil-contaminated soils as a result of petrochemical spills or industrial development. Oil-contaminated soil may affect human and animal health, air and water quality and the production of agronomic, range and forest crops. Furthermore, soil clean-up is often a prerequisite to the development of contaminated sites for other uses.
Spills of crude oil and petroleum products from the vast network of collection, processing, transportation and marketing facilities in North America pose a major risk to the environment. Tens of thousands of kilometers of pipelines and thousands of hectares of well-sites and facilities are needed to collect and process crude oil. Although reported losses of liquid hydrocarbons from these facilities average less than 0.01 percent of production, spills in Alberta, Canada have exceeded 5.times.10.sup.3 m.sup.3 annually in the 1980's (Energy Resources Conservation Board, 1991, Environment Information System, unpublished document, Calgary, Alberta). Gasoline, fuel oil and jet fuel represent 90 percent of the hazardous materials (3.6 billion tons) transported in the United States annually and account for 25 percent of the reported spills.sup.1. The United States Environmental Protection Agency estimates that 35 percent, or 840,000, of underground fuel storage tanks are leaking.sup.2. Also, another 100,000 to 400,000 tanks, used to store other liquid petroleum and chemical substances, may be or have been leaking. 2. Description of the Prior Art
Technologies to clean-up petrochemical spills are expensive and all have numerous limitations with respect to costs, efficacy for treating specific petrochemicals and properties of the contaminated soil. Some treatments, such as landfarming and landfilling, are being reduced or abandoned because of environmental concerns. New technologies are being developed but with the possible exception of thermal desorption, which is inappropriate for removal of low volatile organics, all methods are poorly suited for removing petrochemicals from fine textured soils. In these soils the content of particles having diameters less than 0.1 mm is great enough to alter the engineering properties of the soil.
Fine texture is a property of the majority of soils on the earth's surface which are capable of supporting vascular plants. Most of these soils have 20 percent, or more, of their particles in the less than 0.5 mm diameter size range.sup.3. With the exception of solid phase bioremediation, technologies to treat contaminated fine textured soils generally require that the treated soil be landfilled because alternative treatments impair soil quality, reduce or prevent the growth of plants and increase the likelihood of erosion.
Examples of existing technologies for treating petroleum contaminated soil include: incineration (retorting), thermal desorption, soil washing, bioremediation.
Incineration: This method is very effective for removing organic contaminants but the humic material, necessary for maintenance of topsoil's essential characteristics, is destroyed. Also, the mineralogical properties of the soil are altered, adversely affecting its geotechnical properties. Such treated soil is only suitable for landfill.
Thermal desorption: Suitable for removal of volatile hydrocarbons such as: gasoline, diesel fuel etc. Method is not appropriate for heavy oils, bitumens or asphaltic materials. However, the natural humic matter may be affected, by oxidation, even at low operating temperatures.
Soil Washing: This technique is mostly used for coarse soils and involves scrubbing contaminated soil with water or surfactant solutions. Any fines in the soil are dispersed in the aqueous phase to form a middlings fraction that is difficult to separate and treat.
Bioremediation: Cleaning by this method is slow and requires maintenance of proper drainage, aeration and fertilization of the contaminated soil over long periods of time. Also, soil organisms are not very effective for the treatment of some organic compounds. This treatment may leave the soil in an undesirable hydrophobic condition.
Solvent extraction: This approach can effectively remove contaminants by extraction with a solvent. However, soil fines presents a solids-liquid separation problem.
The National Research Council of Canada has also previously developed a Solvent Extraction/Spherical Agglomeration (SESA) process, described, for example, in U.S. Pat. No. 4,719,008 of 12 Jan. 1988, the Disclosure of which is Incorporated by Reference herein. This process has been used for the extraction of bitumen from oil/tar sands.